This invention relates to firing systems for use with pulverized solid fuel-fired furnaces, and more specifically, to a pulverized solid fuel nozzle tip with a ceramic component for use in such firing systems.
It has long been known in the prior art to employ pulverized solid fuel nozzle tips in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces. By way of exemplification and not limitation in this regard, reference may be had to U.S. Pat. No. 2.895,435 entitled xe2x80x9cTilting Nozzle For Fuel Burnerxe2x80x9d, which issued on Jul. 21, 1959 and which was assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 2,895,435, there is provided a tilting nozzle that is alleged to provide substantially uniform distribution of the fuel-air mixture leaving the tilting nozzle and substantially uniform velocity across the discharge opening of the tilting nozzle into the furnace. To this end, the tilting nozzle includes an inner conduit within an outer conduit. Moreover, a plurality of baffles or division walls are provided within the inner conduit arranged in planes substantially parallel to fluid flow and such as to divide the inner conduit into a multiplicity of parallel channels. These baffles or division walls are designed to be operative to correct the concentration of the air-fuel mixture along the deflecting wall of the inner conduit and the resulting relatively unequal pressure there when the titling nozzle is tilted. Thus, the effect is that as the tilting nozzle is tilted, either upwardly or downwardly, the unequal velocities through the tilting nozzle are made substantially equal by restricting the flow in the high pressure zone present at the inlet end of the inner conduit and encouraging the flow in the low pressure zone also present at the inlet end of the inner conduit.
Another prior art form of a pulverized solid fuel nozzle tip that has been employed in firing systems of the type that are utilized in pulverized solid fuel-fired furnaces is depicted in U.S. Pat. No. 4,274,343 entitled xe2x80x9cLow Load Coal Nozzlexe2x80x9d, which issued on Jun. 23, 1981 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,274,343, there is provided a fuel-fired admission assembly of the type incorporating a split coal bucket having an upper and a lower coal nozzle pivotally mounted to the coal delivery pipe and independently tiltable of each other. Continuing, a plate is disposed along the longitudinal axis of the coal delivery pipe with its leading edge oriented across the inlet end of the coal delivery pipe so that that portion of the primary air pulverized coal stream having a high coal concentration enters the coal delivery pipe on one side of the plate and that portion of the primary air-pulverized coal stream having a low coal concentration enters the coal delivery pipe on one side of the plate and that portion of the primary air-pulverized coal stream having a low coal concentration enters the coal delivery pipe on the other side of the plate. Moreover, the trailing edge of the plate is orientated across the outlet end of the coal delivery pipe such that that portion of the primary air-pulverized coal stream having a high coal concentration is discharged from the coal delivery pipe through the upper coal nozzle and such that that portion of the primary air-pulverized coal stream having a low coal concentration is discharged from the coal delivery pipe through the lower coal nozzle.
Although the pulverized solid fuel nozzle tips that form the subject matter of the above-noted U.S. patents have been demonstrated to be operative for their intended purposes, there has nevertheless been evidenced in the prior art a need for such pulverized solid fuel nozzle tips to be further improved. In this regard, it has been found that pulverized solid fuel deposits, i.e., coal deposits, on and within the pulverized solid fuel, i.e., coal, nozzle tips are problematic from an operational standpoint. That is, such coal deposits on and within the coal nozzle tip have been found to lead to either premature or catastrophic coal nozzle tip failure, depending primarily upon the tenacity of the formed deposits and the rate at which the deposition occurs. To this end, deposition of coal on or within the coal nozzle tip is believed to be caused by a combination of the following three variables: 1) coal composition/type, i.e., slagging, non-slagging, sulfur/iron content, plasticity, etc.; 2) furance/coal nozzle operational settings, i.e., primary/fuel air flow rate/velocity, tilt position, firing rate, etc.; and 3) coal nozzle tip aerodynamics.
Thus, by way of summary, present designs, i.e., prior art forms, of coal nozzle tips have by and large been found to exacerbate the coal deposition problem through the creation of regions of low or negative velocities, i.e., recirculation, that cause slowly moving, xe2x80x9chotxe2x80x9d, coal particles to come in contact with xe2x80x9chotxe2x80x9d coal nozzle tip metal surface. Namely, it has been found that as a result of this interaction, and under requisite thermal conditions that are related to the coal""s plasticity, some of the coal particulate sticks to the plate, thus initiating the deposition process. Moreover, with specific reference to present designs, i.e., prior art forms, of coal nozzle tips, it has been found that regions of low and negative velocities typically occur along the thickness of the nozzle plane platework and in the sharp corners of the primary air shroud.
There has, therefrom, been evidenced in the prior art a need for a new and improved pulverized solid fuel nozzle tip that would address the deficiencies from which present designs, i.e., prior art forms of pulverized solid fuel nozzle tips have been found to suffer. Namely, there has been evidenced in the prior art a need for a new and improved pulverized solid fuel nozzle tip that would be advantageously characterized in the following respects: 1) would minimize low and negative, i.e., recirculation, velocity regions at the exit plane of the pulverized solid fuel nozzle tip, 2) would reduce available deposition surface on the pulverized solid fuel nozzle tip, and 3) would vary the nozzle tip/solid fuel nozzle thermal conditions to keep the xe2x80x9chotxe2x80x9d solid fuel particulate matter from deposition on available metal platework surfaces of the pulverized solid fuel nozzle tip. Such a new and improved pulverized solid fuel nozzle tip accordingly would be effective in controlling the deposition phenomena, from which present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips have been found to suffer. This would be accomplished through the aerodynamic design embodied by such a new and improved pulverized solid fuel nozzle tip coupled with proper adjustment of the controllable operational variables, i.e., fuel air flow rate, etc. As employed herein, the term xe2x80x9ccontrollablexe2x80x9d refers to the fact that solid fuel type and furnace load, and in some, notably retrofit, cases primary air flow rate are typically not controllable operational variables for mitigation of the deposition phenomena.
A common material composition for pulverized solid fuel nozzle tips is stainless steels, typically with relatively high temperature ratings such as, for example, 309 stainless steel. While stainless steel has the desirable material properties of ease of effort in incorporating it into the finished product, toughness, durability, high temperature strength, and ductility, certain material properties of conventional pulverized solid fuel nozzle tips comprised of stainless steel often force operators of pulverized solid fuel combustion facilitates to operate their facilities in a less than optimal economic manner to avoid exceeding the physical limits of such conventional pulverized solid fuel nozzle tips.
Two such limiting material properties are the ability of a stainless steel pulverized solid fuel nozzle tip to maintain its structural integrity at a high temperature (i.e., the maximum operating temperature) and the wear resistance of the pulverized solid fuel nozzle tip. A common maximum operating temperature for a stainless steel pulverized solid fuel nozzle tip is about 2100 degrees Fahrenheit (2100xc2x0 F.) while it is not uncommon that the actual operating temperature of the pulverized solid fuel combustion facility can reach or exceed 2500 degrees Fahrenheit (2500xc2x0 F.). Although there are design and operating approaches which are configured to prevent exposure of the pulverized solid fuel nozzle tip to the actual pulverized solid fuel combustion facility operating temperature such as, for example, providing cooling air within or around the pulverized solid fuel nozzle tip, there is still some risk that the pulverized solid fuel nozzle tip may nonetheless be exposed to temperatures above the recommended maximum operating temperature in spite of the use of such design and operating approaches. For example, in the event that the requisite cooling air which would normally be supplied to protect the pulverized solid fuel nozzle tip is, in fact, not supplied or is only inadequately supplied, the pulverized solid fuel nozzle tip may be exposed to temperatures greater than its recommended maximum operating temperature.
Excess exposure to temperatures beyond its recommended maximum operating temperature may cause a stainless steel pulverized solid fuel nozzle tip to fail during non-maintenance operation of the pulverized solid fuel combustion facilityxe2x80x94in other words, at a time between regularly scheduled maintenance outagesxe2x80x94whereupon the operation of the pulverized solid fuel combustion facility will be disrupted with consequent negative economic impact. The relatively modest wear resistance properties of the stainless steel in a stainless steel pulverized solid fuel nozzle tip may so compromise the pulverized solid fuel nozzle tip that the pulverized solid fuel nozzle tip fails between regularly scheduled maintenance outages, thus leading to the necessity of replacing the pulverized solid fuel nozzle tip at an unscheduled, economically disadvantageous time. While the wear resistance of a stainless steel pulverized solid fuel nozzle tip may be enhanced by measures such as, for example, coating the leading edges of the splitter plates of the pulverized solid fuel nozzle tip with a wear resistant material, such measures add to the manufacturing complexity and the weight of the thus treated pulverized solid fuel nozzle tip, thus detrimentally adding to the costs of the pulverized solid fuel nozzle tip.
In addition to those typical characteristics of a stainless steel pulverized solid fuel nozzle tip which may lead to catastrophic or unplanned operational failure, there are other characteristics of a stainless steel pulverized solid fuel nozzle tip which detract from the desirability of such pulverized solid fuel nozzle tips. For example, depending upon the pulverized solid fuel combustion facility and the type of pulverized solid fuel being combusted, a stainless steel pulverized solid fuel nozzle tip may experience slag build up attributable, in part, to the tendency of slag to bond to the surface of stainless steels. If the slag build up continues, the pulverized solid fuel nozzle tip may ultimately be completely blocked to through flow of the pulverized solid fuel.
To this end, such a new and improved pulverized solid fuel nozzle tip would be advantageously characterized by the fact that certain features were collectively embodied thereby. A first such feature is that the primary air shroud would be recessed. Recessing the primary air platework, i.e., primary air shroud, to within the exit plane of the fuel air shroud would remove this potential deposition surface from the firing zone, i.e., the exit plane of the nozzle tip, and would provide some cooling via the shielding effect of the fuel air shroud. Additionally, a shorter primary air plate, i.e., primary air shroud, would reduce the contact surface for heat transfer thereto and deposition thereon of coal particles. A second such feature is that the splitter plates would be recessed. Recessing the splitter plates along with the primary air shroud to within the exit plane of the fuel air shroud would remove this potential deposition surface from the firing zone, i.e., the exit plane of the nozzle tip, and would provide some cooling via the shielding effect of the fuel air shroud. Additionally, shorter splitter plates would reduce the contact surface for heat transfer thereto and deposition thereon of coal particles. A third such feature is that the fuel air shroud support ribs would be recessed. Recessing the fuel air shroud support ribs would keep the circulation region, and vertical deposition surface normally created by these devices at the exit of the nozzle tip from the firing zone, thus reducing their possible influence in the deposition process. Structurally, recessing the fuel air support ribs would also allow the front portions of the fuel air and primary air shrouds to independently expand reducing thermally induced stress. A fourth such feature is that the trailing edge of the primary air shroud would be tapered. Tapering the trailing edge of the primary air shroud would reduce the recirculation region created by the blunt faced trailing edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips. Such a recirculation region draws hot particulate matter back to the vertical plate surface creating or exacerbating the coal deposition phenomena. Also, such a recirculation region can provide conditions conducive to combustion, thus creating flames within the recirculation region, which raise temperatures and further exacerbate the deposition problem.
To this end, the primary air shroud platework would be tapered at a small enough angle such that neither the fuel air nor the primary air flows separate from the plate thus obviating the creation of additional, unwanted recirculation. A fifth such feature is that the splitter plate ends would be tapered. The splitter plate ends would be tapered to reduce the recirculation region created by the blunt faced trailing edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips, and the shed vortices created by the blunt faced leading edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips. As in the case of the blunt faced trailing edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips, the recirculation region induced by the blunt faced splitter plate of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips draws hot particulate back to the vertical plate surface creating or exacerbating the coal deposition phenomena. Also, such a recirculation region can provide conditions conducive to combustion, thus creating flames within the recirculation region, which raise temperatures and further exacerbate the deposition problem. In addition, the vortices induced by the blunt faced leading edge of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips increase turbulence levels within the primary stream thus exacerbating coal particulate deposition. To this end, the splitter plate edges would be tapered at a small enough angle to avoid primary air separation, which would create additional, unwanted flow recirculation. A sixth such feature is that the fuel air shroud would embody a bulbous inlet. The bulbous inlet of the fuel air shroud would minimize fuel air bypass of the fuel air shroud during tilt conditions which currently occurs with present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips. Moreover, the bulbous inlet would enhance fuel air flow through the fuel air shroud thereby acting to both cool the nozzle tip platework, and thermally blanket the primary air/coal stream to delay ignition, which also provides a tip cooling effect. On the other hand, were the fuel air shroud flow to be allowed to drop severely due to tip bypass, low pressure/velocity regions could be created within the fuel air shroud, leading to reverse flow and particle deposition within this annular region. A seventh such feature is that the primary air shroud exit plane corners would be rounded. Rounding the primary air shroud exit plane corners increases the corner velocities with respect to that found in the ninety degree corners of present designs, i.e., prior art forms, of pulverized solid fuel nozzle tips. Increasing the corner velocities increases the erosion energy for air/coal flowing through this region to help remove active deposits, and otherwise avoid deposition. Also, the rounded corners decrease the available surface for heat transfer from the hot platework to the cooler air/coal mixture for a volume element of air/coal within the tip corner. An eighth such feature is that the fuel air shroud exit plane corners would be rounded. The rounded fuel air shroud exit plane corners, combined with the rounded primary air shroud exit plane corners, provide for higher corner velocities, thus minimizing low velocity regions on the fuel air shroud. In addition, the rounded fuel air shroud exit plane corners assist in achieving a uniform fuel air opening. A ninth such feature is that a uniform fuel air shroud opening (exit plane) would be provided. Providing a uniform fuel air shroud opening provides for uniform fuel air distribution within the nozzle tip. Namely, providing a uniform fuel air shroud opening provides for uniform nozzle tip cooling via the fuel air stream, but also provides for uniform blanketing of the primary air stream for control of ignition position and of NOX emissions. A tenth such feature is that for certain applications wherein minimum NOX emissions and/or minimum carbon in the flyash are criteria that need to be met, it would be possible to provide a version of such a new and improved pulverized solid fuel nozzle tip embodying collectively all of the nine features that have been enumerated hereinabove, which would enable minimum NOX emissions and/or minimum carbon in the flyash to be realized, while yet thereby enabling there to be realized concomitantly therewith minimum fuel deposition and therethrough avoidance of pulverized solid fuel nozzle tip failure occasioned thereby. Moreover, such minimization of NOX emissions and/or minimization of carbon in the flyash would be attainable by providing a version of such a new and improved pulverized solid fuel nozzle tip wherein one or more bluff bodies, each embodying a predefined geometry, are suitably supported in mounted relation at a predetermined location therewithin.
Moreover, irrespective of the dimensions or configuration of the pulverized solid fuel nozzle tip, including the presence or absence of features such as a predetermined recessed spacing of the primary air shroud from the exit plane of the nozzle tip, a tapered profile of the primary air shroud platework, or primary air shroud exit plane rounded corners, a new and improved pulverized solid fuel nozzle tip would be characterized by the fact that it comprises a ceramic material such as, for example, silicon nitride, siliconized silicon carbide (having a silicon content of between about twenty percent (20%) to sixty percent (60%) by weight, mullite bonded silicon carbide alumina composite, and alumina zirconia composites.
It is, therefore, an object of the present invention to provide a new and improved solid fuel nozzle tip for use in a firing system of the type utilized in pulverized solid fuel-fired furnaces.
It is a further object of the present invention to provide such a new and improved solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is comprised of a ceramic material.
It is yet another object of the present invention to provided such a new and improved solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is comprised of a ceramic material from the group of ceramic materials including silicon nitride, siliconized silicon carbide (having a silicon content of between about twenty percent (20%) to sixty percent (60%) by weight, mullite bonded silicon carbide alumina composite, and alumina zirconia composites.
It is another object of the present invention to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that the primary air shroud thereof is recessed.
Yet still another object of the present invention is to provide such a new and improved MRFC solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace that is characterized in that for purposes of attaining therewith minimum NOX emissions and/or minimum carbon in the flyash one or more bluff bodies, each embodying a predefined geometry, are suitably supported in mounted relation at a predetermined location therewithin.
In accordance with one embodiment of the present invention there is provided a solid fuel nozzle tip for use in a firing system of the type utilized in a pulverized solid fuel-fired furnace. The subject solid fuel nozzle tip, in accordance with this one embodiment of the present invention, is constructed so as to be capable of operation as a minimum recirculation flame control (MRFC) solid fuel nozzle tip. To this end, the subject MRFC solid fuel nozzle tip is streamlined aerodynamically to prevent low or negative velocities at the exit of the MRFC solid fuel nozzle tip, which otherwise could provide sites for the deposition thereat of solid fuel particles. As such, the subject MRFC solid fuel nozzle tip is thus effective in eliminating field problems, which heretofore have existed and which have been occasioned by the fact that solid fuel nozzle tip deposits have occurred when certain xe2x80x9cbad slaggingxe2x80x9d solid fuel types, i.e., those having high sulfur/iron content are being fired. Such field problems, in turn, have ultimately resulted in premature failure of the solid fuel nozzle tips embodying prior art forms of construction.
The nature of the construction of the subject MRFC solid fuel nozzle tip, in accordance with this one embodiment thereof, is such that the subject MRFC solid fuel nozzle tip includes fuel air shroud means, primary air shroud means located within the fuel air shroud means, fuel air shroud support means operative for supporting the primary air shroud means within the fuel air shroud means, and splitter plate means mounted in supported relation within the primary air shroud means. The fuel air shroud means embodies a bulbous configuration at the inlet thereof whereby bypassing of the fuel air around the fuel air shroud means during tilt conditions is minimized and whereby the cooling effect of the fuel air flow through the fuel air shroud means is enhanced. In addition at the exit end thereof the fuel air shroud means embodies rounded corners that in turn provide for higher corner velocities thus minimizing low velocity regions on the fuel air shroud means whereat solid fuel particle deposition could occur. With regard to the primary air shroud means, the primary air shroud means at the exit plane thereof is recessed to within the exit plane of the fuel air shroud means whereby the exit plane of the primary air shroud means is removed as a potential deposition surface for solid fuel particles. In addition, the primary air shroud means embodies a tapered trailing edge that is operative to reduce the recirculation region at the trailing edge of the primary air shroud means that might otherwise be operative to draw hot particulate matter back to the trailing edge surface of the primary air shroud means and thereby create or exacerbate thereat the solid fuel particle deposition phenomena. The primary air shroud also embodies rounded exit plane corners that operate to increase velocities in the corners that in turn assist in helping to avoid deposition of solid fuel particles thereat, and in the event such deposition does occur helps in effecting the removal thereof. In addition, the rounded exit plane corners of the primary air shroud means coupled with the rounded exit plane corners of the fuel air shroud means provide the subject MRFC solid fuel nozzle tip with a uniform fuel air shroud opening, which in turn provides for uniform fuel air flow distribution within the subject NRFC solid fuel nozzle tip. Next, as regards the fuel air shroud support means, the fuel air shroud support means is recessed relative to the exit plane of the MRFC solid fuel nozzle tip so as to keep the recirculation region and vehicle deposition surface normally created thereby away from the exit plane of the MRFC solid fuel nozzle tip, thus reducing the fuel air shroud support means"" possible influence in the deposition process. Further, structurally, recessing the fuel air shroud support means also allows the front portion of the fuel air shroud means and the front portion of the primary air shroud means to independently expand and thereby reduce thermally induced stress. Lastly, insofar as the splitter plate means is concerned, the splitter plate means along with the primary air shroud means is recessed, reference having been made hereinbefore to the recessing of the primary air shroud means, to within the exit plane of the fuel air shroud means thereby removing the splitter plate means as well as the primary air shroud as surfaces susceptible to potential depositions arising from the firing zone, i.e., the exit plane of the MRFC solid fuel nozzle tip. Also, such recessing is effective for purposes of providing some cooling via the shielding effect provided by the fuel air shroud means. In addition, such recessing of the splitter plate means results in a shorter splitter plate means thereby reducing the contact surface for heat transfer thereto as well as the contact surface for the deposition of solid fuel particles thereon. Furthermore, the ends of the splitter plate means are tapered but at a small enough angle to avoid primary air separation, which cause the creation of additional unwanted flow recirculation. Such tapering of the ends of the splitter plate means is effective in reducing the recirculation region that has served to adversely affect the operation of prior art forms of solid fuel nozzle tips, which are characterized by the fact that they embody a blunt faced trailing edge, and in reducing the shed vortices that are created by such blunt faced trailing edges. If the splitter plate means were to embody blunt ends, the recirculation region induced thereby would operate to draw hot particulate back thereto and thus would have the effect of creating or exacerbating the solid fuel deposition phenomena. Such a recirculation region is also capable of providing conditions conducive to combustion, thus creating flames within the recirculation region, which would have the effect of raising temperatures and further exacerbating the deposition problem. Moreover, leading edge induced vortices created by blunt faced edges occasion increased turbulence levels within the primary air stream and thus exacerbate solid fuel particulate deposition on such edges, a result that is obviated when tapered edges are employed rather than blunt edges.